Method of inspecting ununiformity of transparent material, apparatus therefor, and method of selecting transparent substrate

ABSTRACT

A laser beam L from a laser  2  is introduced from an introducing surface into a transparent substrate  1  by using mirrors  31  and  32.  The laser beam introduced through the transparent substrate  1  repeats a total reflection on the surfaces (main surfaces and end surfaces) of the transparent substrate  1  and enters a state in which the laser beam is almost confined in the substrate  1.  When an ununiform portion such as a scratch exists on the surface of the transparent substrate  1 , however, total reflecting conditions are not satisfied and the light leaks out of the ununiform portion. The leaked light is formed as an image on a CCD  6  by a lens system  7  and an image process is executed by an image processing apparatus  12.  In a detected image, the ununiform portion in which the scratch or the like exists is brightly seen in a linear or a dot form in a black background, so that the ununiform portion such as a very fine scratch can be detected.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 09/254,849, filedMay 24, 1999 and issuing as U.S. Pat. No. 6,610,994, issuing on Aug. 26,2003, which is a 371 of PCT/JP98/03200 filed Jul. 6, 1997.

TECHNICAL FIELD

The present invention relates to a method of inspecting an opticalununiformity (defect) of a transparent material such as a glasssubstrate serving as a transparent substrate for a photo mask or atransparent substrate for an information recording medium and anapparatus therefore. More particularly, the invention relates to amethod of inspecting an ununiformity of a transparent material and anapparatus therefore, whereby the ununiformity of the transparentmaterial can be detected at a high sensibility and a high speed by usingcharacteristics of a total reflection on the surface of the transparentmaterial and its apparatus.

BACKGROUND ART

In a manufacturing process of a semiconductor integrated circuit, aphoto mask, or the like, a photolithography method is used to form afine pattern. For example, when the semiconductor integrated circuit ismanufactured, a pattern is transferred onto a transparent substratewhich was mirror-finished by polishing at a high precision by using aphoto mask whose pattern was formed by a transparent film (for example,a chromium film). As a method of inspecting the photo mask which can besaid as an original board of the pattern, as shown in a surface stateinspecting apparatus disclosed in Japanese Patent Application Laid-OpenNo. 58-162038 (1983), a method of converging light onto a fine region ona pattern surface and comparing a reflection output and transmissionoutput from the pattern surface has been known.

In recent years, however, in association with a realization of highdensity of the pattern, in addition to the inspection for the patternsurface similar to the above method, a fine defect of the transparentsubstrate itself which was mirror-finished by polishing at a highprecision is also regarded as a target of the defect detection. In theabove-mentioned method, since the light is converged onto the fineregion on the pattern surface, when an inspecting region extends over awide range, it is necessary to scan the light by using some means, along inspecting time is required in proportional to the area of theinspecting region, and a change in light quantity of the reflectionlight and transmission light for the pattern itself and transparentsubstrate is not large depending on the presence or absence of thedefect, so that it is difficult to apply the method to the detection ofthe fine defect on the transparent substrate.

Also with regard to a transparent substrate for an information recordingmedium, from the viewpoints of the formation of an under layer and amagnetic layer having a good crystallinity which are formed on thesurface of the transparent substrate, the low flotation of a magnetichead, and the like with the realization of high density recording, thetransparent substrate having the surface polished at a-high precision isrequired, so that the fine defect of the transparent substrate itself isalso set to a target of the defect detection. However, existing defectinspecting method and apparatus do not necessarily satisfy a request ofthe defect detection.

According to the invention, therefore, in order to solve the aboveproblems, it is an object to provide a method of inspecting anununiformity of a transparent material, whereby an optical ununiformityof the transparent material can be certainly detected at a highprecision and a high speed and an apparatus therefore.

Further, an object of the invention is to provide a method of inspectingan ununiformity of a transparent material, whereby a desired transparentmaterial can be immediately extracted and an apparatus therefore.

FIG. 1 is a schematic constructional diagram showing an embodiment of anapparatus for inspecting an ununiformity of a transparent materialaccording to the invention;

FIG. 2 is a side elevation view in which a part of the transparentsubstrate in FIG. 1 is enlarged;

FIG. 3 is a perspective view showing an example of a folder for holdingthe transparent substrate;

FIG. 4 is an image of a scratch on the surface of a glass substratedetected by the inspecting apparatus in FIG. 1;

FIG. 5 is a diagram showing a light intensity distribution in the widthdirection of the scratch obtained by light information of the image ofFIG. 4 by an image process;

FIG. 6 is an image when the scratch of FIG. 4 is observed by an opticalmicroscope (reflection·bright field);

FIG. 7 is a diagram showing a light intensity distribution in the widthdirection of the scratch obtained from light information of the image ofFIG. 6 by the image process;

FIG. 8 is a graph in which a detection sensibility of the methodaccording to the invention is compared with that of a conventionalmethod and which shows a relation between a signal-to-noise ratio(hereinbelow, referred to as an S/N ratio) and normalized exposing time;

FIG. 9 is a perspective view for explaining a simulation for obtaining abeam locus in the transparent substrate when a laser beam is introducedinto the transparent substrate along one side;

FIG. 10 is a diagram showing an example of a result according to thesimulation of FIG. 9;

FIG. 11 is a graph showing a relation between an incident angle to thetransparent substrate and the number of reflecting times on surfaces inFIG. 10;

FIG. 12 shows graphs of the beam loci showing states of the propagationof the light in the transparent substrate obtained by the simulation ofFIG. 9;

FIG. 13 is a graph showing a relation between the incident angle to thetransparent substrate and the number of reflecting times on the surfacesin another example of the simulation of FIG. 9;

FIG. 14 is a graph showing the relation between the incident angle tothe transparent substrate and the number of reflecting times on surfacesin the other example of the simulation of FIG. 9;

FIG. 15 is a graph showing the relation between the incident angle tothe transparent substrate and the number of reflecting times on surfacesin the other example of the simulation of FIG. 9;

FIG. 16 shows graphs of beam loci showing results of simulating apropagation of the light when a laser beam is introduced to thetransparent substrate;

FIG. 17 shows diagrams showing images such as scratches of thetransparent substrate observed by an ununiformity inspection of theinvention;

FIG. 18 is a perspective view showing a state when laser beams areintroduced from two directions into the transparent substrate;

FIG. 19 is a schematic constructional diagram showing an embodiment inwhich the laser beam is introduced from a corner portion of thetransparent substrate;

FIG. 20 is a perspective view showing an embodiment in which laser beamsare introduced from the same incident position in a plurality ofdifferent directions in the transparent substrate;

FIG. 21 is a flowchart showing an embodiment of an inspecting processesfor separating good and bad transparent substrates from each other;

FIG. 22 is a diagram for explaining the first embodiment in which amethod of selecting a transparent substrate according to the inventionis applied to a glass substrate for a photo mask;

FIG. 23 is a diagram showing a light intensity distribution in the widthdirection of a scratch obtained from light information of an image ofthe scratch on the surface of the glass substrate detected by theinspection of the first embodiment in FIG. 22 by the image process;

FIG. 24 is a diagram for explaining the second embodiment in which themethod of selecting the transparent substrate according to the inventionis applied to a glass substrate for a magnetic disk;

FIG. 25 is a diagram for explaining an inspecting method of the secondembodiment of FIG. 24;

FIG. 26 shows diagrams showing another embodiment for the ununiformityinspection of the invention;

FIG. 27 is a diagram showing a refraction of light on boundary surfaceshaving different refractive indices;

FIG. 28 is a diagram used to obtain total reflecting conditions on thesurface of a transparent material; and

FIG. 29 is a diagram showing a region satisfying the total reflectingconditions under which the light is confined in a rectangulartransparent material by a multiple total reflection.

DISCLOSURE OF THE INVENTION

According to the invention, there is provided a method of inspecting anununiformity of a transparent material by introducing a laser beam intothe transparent material, wherein the transparent material has: at leastone pair of total reflective surfaces in which the laser beam introducedin the transparent material repeats the total reflection and which faceeach other and at least one pair of turning surfaces which are arrangedso as to face each other in the progressing direction of the laser beamthat repeats the total reflection between the total reflective surfacesand progresses and which totally reflect the laser beam and return tothe total reflective surfaces, when an optical path in the transparentmaterial is optically uniformed, the laser beam is introduced in such amanner that the laser beam which propagates in the transparent materialand impinges on the total reflective surfaces and the turning surfacesof the transparent material is propagated so as to totally reflect andrepeat between at least the pair of turning surfaces and the laser beamis spread in an inspecting region which is formed by the propagation andis surrounded by the total reflective surfaces and the turning surfaces,and when an ununiform portion exists in the optical path of the laserbeam which is introduced into the transparent material and propagates,light that leaks out of the total reflective surfaces and/or the turningsurfaces is detected, thereby inspecting an ununiformity of thetransparent material.

When there is no ununiform portion such as a scratch on the surface inthe transparent material, the laser beam introduced through thetransparent material repeats the total reflection on the surface and isconfined in the transparent material (the laser beam is spread), so thatthe light does not substantially leak to the outside (on the totalreflective surfaces and turning surfaces). When the ununiform portionexists in the transparent material, however, the total reflectingconditions are not satisfied and the light leaks out of the surface ofthe transparent material. That is, when the ununiform portion (defect)such as scratch, crack, and stain due to an adhering foreign matterexists on the surface of the transparent material, the light leaks outof the surfaces (total reflective surfaces and turning surfaces) astotal reflective surfaces so long as the optical path is uniform. Inaddition to the ununiformity of the surface of the transparent material,also with respect to the detection of a defect such as internal scratch,crack, or foreign matter such as bubbles or a defect of the glass inwhich a transmission is the same but a reflective index alone isdifferent, which fact is peculiar to the striae of the glass, light isout of the optical path (passage) where the light inherently passes ifit is uniform in the scratch or a portion where the refractive index isdifferent, the total reflecting conditions on the surface are notsatisfied, and the light leaks to the outside of the transparentmaterial, so that it can be detected.

As mentioned above, according to the inspecting method of the invention,since the light is (substantially) confined in the transparent materialby using the geometrical and optical total reflection serving as aphysical critical phenomenon, responses to inspection light in theununiform and uniform portions of the transparent material as aninspecting object are also critical, so that the ununiformity appears asa dramatic contrast. That is, the inspecting method of the invention canbe called as an inspecting method for the defect (ununiformity) by alight confining method. A defect such as a very fine scratch of thetransparent material is observed as light in a shielding case like ablack box leaks out of a pinhole existing on the case.

Incident conditions of the light for repeating the total reflection onthe surface of the transparent material to (substantially) confine thelight in the transparent material are obtained as follows. Theconditions to (substantially) confine the light by the multiple totalreflection in a rectangular transparent material like a transparentsubstrate will now be obtained hereinbelow.

Prior to the obtaining of the conditions to confine the light, first, asshown in FIG. 27, the direction of refraction light when the lightimpinges on a medium such as glass having transparency of a refractiveindex nt from a medium such as air of a refractive index ni is obtained(although a vector is expressed by a normal arrow in the diagram, avector A is expressed as <A> in the document).

As shown in FIG. 27, a refraction beam <Lt> when an incident beam <Li>impinges on a boundary surface between the refractive indices ni and ntis considered. A solvent vector which is perpendicular to the boundarysurface and is directed to the side of an incident medium is set to <N>(unit vector). The vector <Lt> of the refraction beam exists on the flatsurface extended by the vectors <N> and <Li> and can be expressed by alinear connection of <N> and <Li>. That is, it can be expressed asfollows.<Lt>=α<Li>+β<N>  (1)Where, α and β are coefficients. In order to simplify a calculation,when it is assumed that the vectors <Li> and <Lt> are set to unitvectors, the following equations are satisfied.<Li>·<Li>=1, <Lt>·<Lt>=1  (2)

When a law of refraction (Snell's law) is applied to the refraction onthe boundary surface, since an incident angle is θi and a refractiveangle is θt,sin θt=(ni/nt)sin θi  (3)When θi and θt are expressed by vectors, $\begin{matrix}\begin{matrix}{< {Li} > {\cdot {< N>={{< {Li} > {{{\cdot {{< N >}}}{\cos\left( {\pi - {\theta\quad i}} \right)}}}}}}}} \\{{= {{- \cos}\quad\theta\quad i}}\quad}\end{matrix} & (4) \\\begin{matrix}{< {Lt} > {\cdot {< N>={{< {Lt} > {{{\cdot {{< N >}}}{\cos\left( {\pi - {\theta\quad t}} \right)}}}}}}}} \\{{= {{- \cos}\quad\theta\quad t}}\quad}\end{matrix} & (5)\end{matrix}$

An object which will now be obtained is the refraction beam vector <Lt>.When α and β of the equation (1) are determined, <Lt> is decided.Therefore, when α and β are expressed by using ni, nt, and θi from theequations (1) to (5),

α=ni/nt$\beta = {{- \left\{ {1 - {\left( {n\quad{i/n}\quad t} \right)^{2}\left( {1 - {\cos^{2}\theta\quad 1}} \right)}} \right\}^{\frac{1}{2}}} + {\left( {n\quad{i/n}\quad t} \right)\cos\quad\theta\quad i}}$

The direction of the refraction beam <Lt> to the incident beam <Li> isdetermined.

Subsequently, conditions under which the refraction beam introduced inthe predetermined direction as mentioned above repeats the totalreflection on the surface of the rectangular transparent material and isconfined in the transparent material will now be obtained.

First, an xy plane is considered as a first boundary surface of thetotal reflection. As shown in FIG. 28, when it is assumed that a normalvector in the positive direction of a z axis is set to <Nz>=(0, 0, 1)and an incident vector <L1> (unit vector)=(L1 x, L1 y, L1 z) impinges onthe xy plane at the incident angle θi, the following equation issatisfied.<Li>·<Nz>=cos(π−θ1)When the equation is expanded,L1 z=−cos θ1Since |<L1>|=1, the following equation is satisfied under conditionsthat when θ1≧θc.L1 x ²+L1 y ²+cos²θ1=1  (6)

It is sufficient that the incident vector which can totally reflect onthe xy plane of the first boundary surface exists on the outside of acircular cone in FIG. 28 obtained by rotating a straight line whichforms a critical angle θc with the z axis around the z axis. The vectorswhich can satisfy the above-mentioned conditions infinitely exist.

Since the vector reflects on the xy plane, the vector <L2> aftercompletion of the reflection is as follows. < L2 >  = (L1x, L1y, −L1z)   = (L1x, L1y, cos   θ  1)

Further, it is necessary to consider conditions under which the vector<L2> totally reflects on a second boundary surface. When it is assumedthat the second boundary surface is set to an xz plane and the vectorimpinges on the xz plane at an incident angle θ2,<L2>·<Ny>=cos(π−θ2)Where, <Ny> denotes a unit vector (0, 1, 0) which is directed to thepositive direction of a y axis. When the above equation is expanded,L1 y=−cos θ2From the above equation and equation (6), it is necessary to satisfy thefollowing equation under conditions that when θ1, θ2≧θc.L1 x ²=1−cos²θ1−cos²θ2  (7)

Since the vector reflects on the xz plane, a vector <L3> aftercompletion of the reflection is as follows.<L3>=(+{1−cos²θ1−cos²2}^(1/2)·cos θ2, cos θ1)When the vector <L3> totally reflects on a third boundary surface, theconfinement of the light is succeeded. When the third boundary surfaceis set to a yz plane and the vector impinges on the yz plane at anincident angle θ3,<L3>·<Nx>=cos(π−θ3)Where,Nx=(1, 0, 0). Therefore, the above equation is expanded to L3x=−cos θ3, so that it is necessary to satisfy the expression of+{1−cos²θ1−cos²θ2}^(1/2)=−cos θ3, that is,cos²θ1+cos²θ2+cos²θ3=1  (8)

sin(π/2)=(nt/ni)sin θc at the boundary angle θc. Therefore, whenni=1.00, nt=1.47, since sin θc=1/1.47, the boundary angle θc=42.9°.

When θ1=θ2=θ3, according to the equation (8), 3 cos² θ1=1 and θ1=54°,such an equation of θ1≧54°>θc=42.9° is sufficiently satisfied. A marginhaving a degree of freedom in the above-mentioned conditions can beutilized as a degree of freedom which allow the total reflection to moreoccur on a specific reflective surface. As for substrate glass, it issufficient to select <L1> so as to reduce the reflection at the endsurface.

As mentioned above, the conditions under which the light is confined bythe multiple total reflection are as follows.cos²θ1+cos²θ2+cos²θ3=1cos θc≧cos θ1, cos θ2, cos θ3>0

When θc=42.9°, cos θc=0.73. Therefore, when a region where θ1, θ2, andθ3 are satisfied is shown in the diagram by using cos θ1, cos θ2, andcos θ3 as axes of coordinates, as shown in FIG. 29, a curved surface 60serving as a surface of a sphere whose radius is equal to 1 in a cubewhose side is equal to 0.73 is the region to satisfy the totalreflecting conditions.

Although the total reflecting conditions with respect to the rectangulartransparent material like a transparent substrate has been derived, itis also possible to decide the conditions of the incident angle of thelight by a method similar to the above in a general form which will bedescribed hereinlater. The conditions of the incident angle of the lightwhich is introduced into the transparent material in the embodiment,which will be described hereinlater, are derived on the basis of theabove.

In the method of inspecting the ununiformity of the invention, thetransparent material can take any form of a square (rectangular) plate,a circular plate, an annular plate, a lens whose curved surface has alarge curvature, a sphere, a polyhedron, a column, a cylinder, and apolyhedral column. When it is possible to realize a state where thelight repeats the reflection of the number of predetermined times ormore in at least the inspecting region of the transparent material andthe light is confined in the transparent material, the form of thetransparent material is not limited.

As mentioned above, in order to confine the light in the transparentmaterial, it is preferable that the transparent material has at leastone pair of total reflective surfaces In which the light introducedthrough the transparent material repeats the total reflection and whichface each other and at least one pair of turning surfaces which arearranged so as to face each other in the progressing direction of thelaser beam that repeats the total reflection between the totalreflective surfaces and propagates and which totally reflect and returnthe laser beam to the total reflective surfaces. Particularly, theturning surfaces are important to confine the light in the transparentmaterial. The turning surfaces have to be provided so as to face eachother in the progressing direction of the light and it is necessary toprovide at least one pair. The reason is that if the light does notrepeat between the turning surfaces, the light cannot be confined.

As for the introduction of the laser beam, it is necessary to introducethe laser beam in such a manner that the laser beam introduced throughthe transparent material is propagated so that light which propagates inthe transparent material and impinges on the total reflective surfacesand turning surfaces totally reflects and repeats between at least thepair of turning surfaces and the laser beam is spread in an inspectingregion which is formed by the propagation and is surrounded by the totalreflective surfaces and the turning surfaces. That is, when the opticalpath in the transparent material is optically uniform, the laser beam isintroduced in such a manner that the light introduced through thetransparent material is propagated while repeating the total reflectionbetween the total reflective surfaces which face each other, the lightimpinges on a turning surface (a) and totally reflects, after that, thelight further repeats the propagation in the transparent material andtotally reflects between at least one of turning surfaces (b), (c), . .. except for the turning surface (a), and the light propagated in theturning surface (a) is again returned. In this case, the optical path inthe transparent material is optically uniform. Even if there is noununiform portion, the total reflection is repeated between the totalreflective surfaces and turning surfaces. Since there is no singularpoint that the light leaks on the total reflective surfaces and turningsurfaces (except for the region to introduce the laser beam), theconfinement of the light is (substantially) realized. In case of onepair of the turning surfaces, the light is confined on one certain planeof the transparent material. In case of a plurality of pairs of theturning surfaces, the confinement of the light is executed in almost thewhole region of the transparent material.

In order to (substantially) confine the light in the transparentmaterial, it is sufficient to introduce the laser beam so that such asingular point that the laser beam geometrically and optically leaksdoes not substantially exist on the total reflective surfaces and theturning surfaces. Since it is “geometrically and optically”, light suchas Rayleigh scattering light derived by an optical change due to afeature that is peculiar to the transparent material is not consideredas an introduced laser beam here. The reason why “such a singular pointthat the laser beam leaks does not substantially exist” is that a casewhere after the laser beam repeats the total reflection many timesbetween the total reflective surfaces and the turning surfaces, a veryslight laser beam does not satisfy the total reflecting conditions andleaks out of the transparent material due to an influence of a spreadangle of the laser beam itself. Therefore, it is “substantially” inconsideration of the case is considered. Since the leak light due to thespread angle of the laser beam itself leaks in the direction along thesurface of the transparent material, it is not detected by detectingmeans, so that it has no effect on a detecting sensitivity.

In order to confine the light in the transparent material, when it isassumed that the refractive index of the transparent material to awavelength λ of the laser beam which is introduced is set to nt, therefractive index of the external medium which is come into contact withthe transparent material is set to ni, and an angle of the light whichimpinges on the total reflective surfaces and turning surfaces is set toθik (k denotes a position where the laser beam impinges on the totalreflective surfaces and turning surfaces after it is introduced throughthe transparent material and the incident positions are is sequentiallyset as k=1, 2, . . . ), it is sufficient to introduce the laser beam sothat θik is equal to a critical angle θ expressed as sin θ=ni/nt or morein the total reflective surfaces and turning surfaces.

As for the surface of the transparent material, with regard to a form inwhich it is difficult to confine the light by the total reflection (forexample, a form whose curved surface has a large curvature), theununiformity can be inspected by propagating the light so as to repeatthe total reflection on at least two pairs of surfaces (at least onepair of total reflective surface and at least one pair of turningsurfaces) which are provided on the outside of the transparent materialand which face each other. Specifically speaking, the inspection can beexecuted in such a manner that a transparent vessel havingmirror-finished surfaces is used, a transparent material is insertedinto a medium (a liquid or the like) in the vessel, which has arefractive index larger than that of an external medium of the vessel,and the laser beam is introduced so as to repeat the total reflection onthe external surface of the vessel and propagate.

It is preferable that by introducing the laser beam so that all of thelaser beams totally reflect on at least the total reflective surfacesand turning surfaces on which the laser beam introduced through thetransparent material first impinges, the light satisfying the totalreflecting conditions can be precisely introduced through thetransparent material. In a case other than the above, for example, whenscattered light propagates in the transparent material on the surface tointroduce the laser beam, since loci of (a plurality of) beams whichwill propagate in the transparent material cannot be expected, it isdifficult that almost all of the introduced laser beams which impingeson the total reflective surfaces and turning surfaces totally reflect,so that the confinement of the light as shown in the invention is notrealized.

It is desirable that an introducing surface to introduce the laser beamis provided in a portion sandwiched by one certain total reflectivesurface and at least one turning surface. As for the introduction of thelaser beam to the transparent material, the laser beam can be introducedfrom the total reflective surface or turning surface other than theintroducing surface. In this case, however, as an entrance window tointroduce the laser beam, an optical member made of a material havingsubstantially the same refractive index as that of the transparentmaterial has to be attached by an adhesive agent or the like, so that ittakes much time. Since the optical member is attached to the totalreflective surfaces or turning surfaces serving as inspecting regions,the light propagated in the transparent material does not satisfy thetotal reflecting conditions in the attached portion and the light leaks,so that the inspection cannot be substantially executed. Therefore, itis not preferable.

As mentioned above, when the introducing surface to introduce the laserbeam exists, as specific introducing means, the laser beam is introducedso as to emit the introduced laser beam into only the introducingsurface and a surface in which an angle formed between the surface andintroducing surface is almost equivalent to an angle formed between thesurface and total reflective surface, thereby realizing the lightconfinement referred to in the present invention.

It is desirable that at least the introducing surface of the transparentmaterial to introduce the laser beam is mirror-polished. The laser beamis introduced so that the total reflection is repeated at the totalreflective surfaces and turning surfaces and the light is confined. Whenthe introducing surface is mirror-polished, however, the introducedlaser beam is not influenced due to a diffusion by the introducingsurface and is propagated as parallel light as it is. Consequently, allof the light which impinges on the total reflective surfaces and turningsurfaces are totally reflected, so that responses of the inspectionlight in the ununiform and uniform portions of the transparent materialbecome more critical and the contrast is improved. Preferably, it isdesired that the whole surface (total reflective surfaces, turningsurfaces, and introducing surfaces) of the transparent material ismirror-polished.

It is assumed that the size of the total reflective surface is set to L,the width of introducing surface is set to d, the refractive index ofthe transparent material for the wavelength λ of the laser beam is setto nt, the refractive index of the external medium which is come intocontact with the transparent material is set to ni, a beam diameter ofthe laser beam is set to φ, the angle of the light incident on the totalreflective surfaces and the turning surfaces is set to θik (k denotesthe position where the laser beam first impinges on the total reflectivesurfaces and turning surfaces after the laser beam is introduced throughthe transparent substrate and the incident positions are sequentiallyset as k=1, 2, . . . Particularly, an angle of the light which firstimpinges on the total reflective surface or turning surface after thelaser beam is introduced is set to θi.), and the number of reflectingtimes at the total reflective surfaces is set to m. When m is expressedby a function of L, d, nt(λ), ni, φ, and θ1, it is preferable thatconditions of at least one of L, d, nt(λ), ni, φ, and θ1 are determinedso that m is equal to a reference set value or more in a range whereeach θik is equal to the critical angle θ or more and the laser beam isintroduced from the introducing surface of the transparent material.

When the light introduced through from the introducing surface repeatsthe propagation in the transparent material and the light impinges onthe introducing surface again, since the light of the critical angle θor less impinges, the light leaks. Therefore, when it is desired thatthe total reflection is more performed in the transparent material (whenit is desired to increase m), it is sufficient to reduce such aprobability that the light leaks out of the introducing surface.Actually, the beam locus is obtained by a simulation, the lightintroduced through the transparent material repeats the total reflectionat the total reflective surfaces and turning surfaces and propagates, sothat the width d of the introducing surface that the number ofreflecting times until the light leaks out of the introducing surfaceincreases is determined. Specifically speaking, it is sufficient thatthe width d of the introducing surface is reduced. Although the width dof the introducing surface is also limited by a beam diameter of thelaser beam and a process for the transparent material, it is desirablethat d is equal to 0.4 mm or less, preferably, 0.2 mm or less. When itis extremely reduced (it is smaller than 0.1 mm), since a break easilyoccurs on an interface between the total reflective surfaces and theintroducing surface and an interface between the turning surfaces andthe introducing surface, it is not preferable.

By selecting the refractive index nt (or wavelength λ of the laser beam)of the transparent material, m (the number of reflecting times on thetotal reflective surfaces) can be adjusted. Specifically speaking, sincethere is a case where the quality of material of the transparentmaterial is limited in accordance with the use of the transparentmaterial, so that it is preferable to select the wavelength λ of thelaser beam. The wavelength of the laser beam in which an absorption tothe transparent material is little is preferable. When the absorption islarge, since there is a possibility that not only a detectingsensitivity of the ununiformity decreases but also the transparentmaterial itself is broken, it is not preferable. The wavelength of thelaser beam also exerts an influence onto a resolution of theununiformity (scratch on the surface or the like). Since the maximum ofthe resolution of the ununiformity becomes the wavelength λ of the laserbeam, when it is desired to resolve and detect such a fine defect thatthe width of a scratch of a glass substrate for an electronic device isequal to 1 μm or less as an image, the wavelength of the laser beam isset to 1 μm or less.

When the transparent material has a certain specific form (for example,when the total reflective surfaces are perpendicular to the turningsurfaces or the like), since the angle at which the beam impinges oneach of the total reflective surfaces and turning surfaces has apredetermined relation with the angle θ1 at which the laser beam firstimpinges on the total reflective surfaces after completion of theintroduction, m (the number of reflecting times on the total reflectivesurfaces) can be adjusted by properly adjusting the angle θ1. In fact,after the conditions of the transparent material (width d and refractiveindex nt of the introducing surface) and the conditions of the laserbeam (wavelength λ and beam diameter φ) are determined, θ1 is adjustedso as to be equal to the reference design value m which permits thetransparent material to be filled with light or more, and the laser beamis introduced. Generally, however, there is not a little variation insize (length or the like) of the total reflective surface depending on adifference between processing precisions. When the size of the totalreflective surface of the transparent material having the variation isgrasped every inspecting sample and the inspection is then performed, ittakes an extensive time, so that it is not practicable. Therefore, thelaser beam which is introduced through the transparent material can bedetected at a high sensitivity and a high speed by fluctuating theincident angle within a range where the total reflection is caused andimpinging the beam, so that the inspecting method having a high utilitycan be realized.

It is preferable that the total reflective surfaces and turning surfacesof the transparent material have such a relation as to cross at rightangles to each other. With such a construction, the introduced laserbeam easily enters a state where it repeats the total reflection on thetotal reflective surfaces and turning surfaces and is confined in thetransparent material. In face, the inspection for the wide region of thetransparent material can be simultaneously executed, so that a highspeed inspection can be realized. That is, the reason is that theincident angle of the light is the same on at least the pair of totalreflective surfaces on which the introduced laser beam repeats the totalreflection and, on at least the pair of turning surfaces as well, theincident angle of the light is the same, so that the light is propagatedso as to have a predetermined relation (when the incident angle of thelight on the total reflective surface is set to θ, the incident angle ofthe light on the turning surface is equal to 90°−θ).

In the inspecting region of the transparent material sandwiched by onecertain pair of the total reflective surfaces and one certain pair ofturning surfaces, the ununiformity in one certain plane in theinspecting region filled with the light due to such a fact that thelight is propagated in the transparent material is inspected and, afterthat, the ununiformity of the inspecting region is inspected byrelatively moving the plane of the inspection in the direction in whichthe inspecting region is filled with the light to the transparentmaterial, so that the inspecting method can be simplified and it ispreferable.

Although the concept of the general light confinement of the inventionhas been explained, when the invention is more specifically embodied,there is provided a method of inspecting an ununiformity of atransparent material by introducing a laser beam through the transparentmaterial, wherein the surface of the transparent material has at leastone pair of main surfaces which are parallel to each other, at least onepair of end surfaces which intersect the main surfaces, and chamferportions sandwiched by the main surfaces and end surfaces, when anoptical path in the transparent material is optically uniform, a laserbeam is introduced in such a manner that light which propagates in thetransparent material and impinges on the main surfaces and the endsurfaces of the transparent material is propagated so as to totallyreflect and repeat between at least the pair of end surfaces and thelaser beam is spread in an inspecting region which is formed by thepropagation and is surrounded by the main surfaces, the end surfaces,and the chamfer portions, and when an ununiform portion exists in theoptical path of the light introduced and propagated in the transparentmaterial, the light that leaks out of the main surfaces and/or endsurfaces is detected, thereby inspecting the ununiformity of thetransparent material.

In this instance, the main surfaces, the end surfaces, and each of thechamfer portions correspond to the foregoing total reflective surfaces,the turning surfaces, and the introducing surface. As a representativeform having the main surfaces, end surfaces, and chamfer portions, asquare (rectangular) plate, a circular plate, an annular plate, or thelike can be mentioned. In this case, the introduced laser beam easilyenters a state where the laser beam repeats the total reflection on themain surfaces and end surfaces and is (substantially) confined in thetransparent material. Actually, since a wide region of the transparentmaterial can be simultaneously inspected and a high speed inspection canbe realized, it is preferable. That is, the reason is as follows. Eachincident angle of the light on the main surfaces on which the introducedlaser beam repeats the total reflection is the same and each incidentangle of the light which impinges on the end surfaces is also the same.Since the light is propagated so that those incident angles keep apredetermined relation (when it is assumed that the incident angle ofthe light which impinges on the main surfaces is set to θ, the incidentangle of light which impinges on the end surfaces is equal to 90°−θ),the light confinement is (substantially) satisfied by merely setting sothat the incident angle to the main surfaces on which the light firstimpinges after it introduces through the transparent material is largerthan a critical point and the incident angle to the end surfaces islarger than the critical point.

As specific means for introducing the laser beam, the laser beam isintroduced so that a singular point that the laser beam geometricallyand optically leaks out of the main surfaces and end surfaces does notsubstantially exist. Further, the laser beam is introduced so that theintroduced laser beam is emitted from only the chamfer portions.

It is also preferable that the transparent material serving as an objectof the ununiformity inspecting method is made of glass. The quality ofmaterial of the transparent material is decided in accordance withvarious uses. In case of glass, there are advantages that it is hard, aremarkably smooth surface can be obtained by mirror-polishing, lightpermeability is good, and the like.

When the transparent material is a glass substrate for an electronicdevice, the ununiformity inspecting method exhibits the effect stillmore. Since a glass substrate having a surface polished at a highprecision is required in association with the realization of highdensity of a pattern in recent years, the ununiformity inspecting methodis effective in inspecting the ununiformity such as fine scratch orstriae of the substrate which becomes an adverse influence on aformation or an exposure of the pattern.

When the transparent material is a glass substrate for an informationrecording medium, the ununiformity inspecting method exhibits the effectmore and more. Since the transparent substrate having the surfacepolished at a high precision is required in association with therealization of high density recording and low flotation of a magnetichead in recent years, the ununiformity inspecting method is effective ininspecting the ununiformity such as a scratch on the substrate surfacewhich becomes an adverse influence onto the realization of high densityrecording and low flotation of the magnetic head.

Similar to the glass substrate for the electronic device, glasssubstrate for the information recording medium, or glass substrate for aliquid crystal display, when the material is formed so as to have mainsurfaces which are parallel to each other and end surfaces which areperpendicular to the main surfaces and so that introduced light repeatsthe total reflection and is confined in the transparent material,actually, a wide region of the transparent material can besimultaneously inspected, so that the inspection can be performed at ahigh speed.

An apparatus for inspecting an ununiformity of a transparent materialaccording to the invention is to embody the above inspecting method andis an apparatus for inspecting the ununiformity of the transparentmaterial by introducing a laser beam through the transparent material,comprising: illuminating means (irradiating means) for introducing alaser beam through the transparent material; and detecting means fordetecting light which leaks out of the transparent material, wherein thetransparent material has an introducing surface for introducing thelaser beam through the transparent material and at least two pairs ofsurfaces on which the introduced laser beam repeats a total reflectionand which face each other, and the illuminating means is arranged sothat a laser beam emitted from the illuminating means is introduced fromthe introducing surface, when an optical path in the transparentmaterial is optically uniform, light which propagates in the transparentmaterial and impinges on the surfaces of the transparent material ispropagated so as to totally reflect and repeat at least one pair ofsurfaces among the above-mentioned surfaces, and the laser beam isspread in an inspecting region which is formed by the propagation and issurrounded by at least the two pairs of surfaces. With such aconstruction, the inspection of the ununiformity of the transparentmaterial can be automatically executed, an inspecting time can bereduced, and a reliability of the inspection can be improved.

In the inspecting apparatus, it is preferable that angle adjusting meansfor changing an incident angle of the laser beam to the transparentmaterial is provided for the illuminating means. The angle adjustingmeans adjusts the incident angle so that the laser beam introducedthrough the transparent material repeats the total reflection on thesurface of the transparent material and the light is confined and isalso used when the incident angle is fluctuated within a range where thetotal reflection occurs in order to absorb a variation in size due to adifference between processing precisions of the transparent material.

As representative angle adjusting means, a mirror can be mentioned. Themirror is arranged between the illuminating means (for example, a laser)and the transparent material and adjusts the incident angle for thetransparent material. In addition to the mirror, it is also sufficientthat angle adjusting means for changing an angle of the illuminatingmeans to the transparent material is provided for the illuminating meansitself or angle adjusting means is provided for a folder for holding thetransparent material. Means for adjusting and fluctuating the incidentangle by using an acousto-optical effect of an ultrasonic beam such asan acousto-optical polariscope can be also used.

In the inspecting apparatus, it is preferable to provide moving(scanning) means for moving (scanning) an incident position of the laserbeam on the transparent material It is because the whole region of thetransparent material can be exhaustively and automatically inspected.For example, illuminating means such as a laser and angle adjustingmeans such as a mirror are mounted on the same table. By attaching adriving apparatus to the table, consequently, they can be sequentiallymoved along one side of the transparent material or by attaching adriving apparatus to the folder for holding the transparent material,the folder can be moved.

In the inspecting apparatus, it is preferable that the transparentmaterial and detecting means are integratedly and relatively moved forthe illuminating means. When a detecting region of the detecting meansis larger than the inspecting region, the transparent material isrelatively moved for the illuminating means, so that the inspection ofthe ununiformity can be performed. However, since the inspecting regionis generally larger than the detecting region of the detecting means,the transparent material and detecting means are integratedly andrelatively moved for the illuminating means. When they are relativelymoved, it is also possible that the illuminating means, namely, anilluminating optical system such as a laser is fixed and the transparentmaterial and/or detecting means are moved by using the driving apparatusor the like, or the transparent material and/or detecting means arefixed and the illuminating optical system such as a laser is moved.

In the inspecting apparatus, it is preferable that the detecting meanshas an image pickup camera having an image pickup device (CCD or thelike) and a lens for forming, as an image onto the image pickup camera,the light which leaks out of the transparent material, and the imagepickup camera and/or lens are relatively moved in the depth directionfor the transparent material. By relatively moving the image pickupcamera and/or lens in the depth direction for the transparent material,a focusing of the image pickup camera can be performed, so that accurateinformation of the ununiformity (scratch on the surface, internalstriae, bubbles, or the like) in the thickness direction of thetransparent material can be obtained. For example, the detecting meanssuch as image pickup camera and lens are fixed and the transparentmaterial, laser, and mirror are integratedly moved in the depthdirection of the detecting means. On the contrary, it is also possiblethat the transparent material, laser, and mirror are fixed and thedetecting means such as image pickup camera and lens are moved.

In the inspecting apparatus, it is desirable to provide discriminatingmeans for discriminating the presence or absence, kind, and size of theununiformity of the transparent material on the basis of the informationdetected by the detecting means.

With respect to the presence or absence, kind (scratch or crack on thesurface portion, striae or foreign matter in the inside, or the like),size (area, length, width, depth, region, or the like) of theununiformity which previously exists in the transparent material,(image) information of the light which leaks out of the surface, arelation (information) between (a light quantity of the leak light,luminance, intensity distribution, depth from the surface, or the like)are accumulated in a computer or the like and (image) information of thelight detected by the inspection is compared with the accumulatedinformation, so that the kind and size of the ununiformity of thetransparent material can be discriminated. As mentioned above, bydiscriminating the presence or absence, kind, and size of theununiformity of the transparent material, a desired transparent materialcan be extracted. Therefore, for example, a glass substrate having anununiformity that has an influence at the time of a formation of apattern or an exposure for a transferring object can be eliminatedbefore the following process after completion of the inspection or canbe returned to a re-polishing process, so that the productivity can beimproved.

In the ununiformity inspecting apparatus, it is preferable that as forthe information of the detected light, when the light detected by theCCD is converted to an SIN ratio (10·log₁₀(S/N)) for normalized exposingtime of the CCD and is processed, the S/N ratio (10·log₁₀(S/N)) is equalto 4.8 dB or more so long as the normalized exposing time is equal to0.025 or more. In this instance, the normalized exposing time is definedas (exposing time of the CCD)/(maximum exposing time of the CCD until asignal of a background reaches (20000/4095)×100 electrons). Theforegoing normalized exposing time can be freely set due to measuringconditions or inspecting apparatus.

The reason is that when the S/N ratio is equal to 4.8 dB or more (anamount of the signal for the background is equal to a value that isthree times as high as that of noises), the value is an image processingpossible level which is generally known, so that the presence orabsence, kind, and size of the ununiformity of the transparent materialcan be accurately discriminated.

A method of selecting a transparent substrate according to the inventionis characterized by comprising the steps of: preparing a transparentsubstrate having an introducing surface to which a laser beam isintroduced, at least one pair of main surfaces on which the introducedlaser beam repeats a total reflection and which face each other, and atleast one pair of end surfaces provided so as to face each other in theprogressing direction of light; introducing the laser beam from theintroducing surface in such a manner that when an optical path in thetransparent substrate is optically uniform, light which propagates inthe transparent material and impinges on the main surfaces and the endsurfaces of the transparent material is propagated so as to totallyreflect and repeat between at least the pair of end surfaces and thelaser beam is spread in an inspecting region which is formed by thepropagation and is surrounded by the main surfaces and the end surfaces;detecting light which leaks out of the main surfaces and/or end surfaceswithout being totally reflected; and comparing the detected Informationwith information which has previously been stored and which correspondsto the presence or absence, kind, and size of an ununiformity existingin the transparent substrate, thereby selecting the transparentsubstrate.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will now be described hereinbelow by usingthe drawings. FIG. 1 is a schematic constructional diagram showing anembodiment of an apparatus for inspecting an ununiformity of atransparent material according to the invention.

In FIG. 1, reference numeral 1 denotes a transparent substrate made ofglass such as optical glass serving as an inspecting object. As shown inFIG. 2, the transparent substrate 1 has parallel planes which face eachother and are constructed by main surfaces (surface and rear surface) Hand end surfaces (T planes and C planes as chamfer portions). Everyplane is mirror-polished and, after that, it is cleaned. The mainsurfaces (surface and rear surface) have a role that the laser beamintroduced through the transparent substrate repeats a total reflectionand propagates and have a function as a total reflective surface. Theend surfaces (T planes) are arranged so as to face each other in theprogressing direction of the light, allow the light which repeated thetotal reflection on the main surfaces and propagated to repeat betweenmirror surfaces which face each other, and have a function as a turningsurface for turning the introduced and propagated light. The C planesare planes sandwiched by the main surfaces and end surfaces (T planes).Generally, in the C plane, since a fine scratch on the surface hardlybecomes a problem, the plane is not regarded as an object of theinspecting region. In the invention, it has a function as an introducingsurface for introducing the laser beam.

In this instance, every surface of the main surfaces (surface and rearsurface) as total reflective surfaces, end surfaces (T planes) servingas turning surfaces, and C planes serving as introducing surfaces ismirror polished. Particularly, such a fact that the introducing surfacesare mirror polished has a sense in the light confinement of theinvention. That is, by mirror-polishing the introducing surfaces, theintroduced laser beam propagates as almost parallel light withoutsubstantially being scattered, so that it is possible to adjust almostall of the light which impinges on the main surfaces and end surfaces soas to be totally reflected. When the introducing surfaces are notmirror-polished, the light is scattered on the introducing surfaces, thelight of a plurality of directions propagates, and every beam locuscannot be expected, so that the light confinement of the invention isnot satisfied. Although explanation is made with respect to such a factthat the introducing surfaces are mirror-polished, so long as the laserbeam can be introduced so that all of the laser beams are totallyreflected on the main surfaces on which the laser beam that isintroduced into the substrate at least first impinges, it is unnecessaryto mirror-polish. For example, by coating matching oil or the likehaving the same refractive index as that of the substrate in order toform a pseudo mirror surface onto the introducing surfaces as mirrorsurfaces, the light confinement of the invention is also realized.

In order not to impede the total reflection on the surface and to easilyexecute the inspection of the leak light, the transparent substrate 1 ishorizontally held by a folder so as to reduce a contact portion aslittle as possible. FIG. 3 shows an example of the folder of thetransparent substrate 1. A folder 20 has a rectangular framing form forholding the transparent substrate 1. Receiving portions 21 forsupporting corner portions of the bottom surface of the transparentsubstrate 1 are formed at four corners on the inner side of the bottomof the folder 20. Spheres 22 for supporting the transparent substrate 1at dots so as to be come into contact with it are placed.

Illuminating means for introducing the laser beam to inspect anununiformity from the side surface of the transparent substrate 1 isprovided for the transparent substrate 1. The illuminating means has: alaser 2 as a light source for emitting an illuminating light; andmirrors 31 and 32 for allowing the laser beam to illuminate atpredetermined position and angle of the C plane. The laser 2 and mirrors31 and 32 are placed on a table 5 on which a driving apparatus 4 formoving the laser beam in parallel with the direction of a side 1 a ofthe transparent substrate 1. In the embodiment, in order to relativelymove the laser 2 and mirrors 31 and 32 for the transparent substrate 1and detecting means, the transparent substrate 1 and detecting means arefixed and the laser 2 and mirrors 31 and 32 can be integratedly moved bythe driving apparatus 4. (It is also sufficient that the drivingapparatus is attached to the transparent substrate 1 and detectingmeans, the laser 2 and mirrors 31 and 32 are fixed, so that thetransparent substrate 1 and detecting means can be integratedly moved.)In order to detect the ununiformity in the thickness direction of thetransparent substrate 1 by executing a focusing of a CCD when the laserbeam is introduced, the table (not shown) on which the laser 2 andmirrors 31 and 32 are placed can be moved in the directions of X, Y, andZ. The mirrors 31 and 32 are used for a fine adjustment of an angle orthe like. It is also sufficient to directly irradiate the laser beamonto the substrate 1 from the laser 2 without using the mirrors 31 and32. Incident angle adjusting means can be also provided so as to enablethe laser beam introduced through the transparent substrate 1 to impingeby fluctuating the incident angle in a range where the laser beam causesthe total reflection. As incident angle adjusting means, means having amechanism for automatically adjusting angles of the mirrors 31 and 32 bya control of a computer or the like or means such as an acousto-opticalpolariscope for allowing the incident angle by using an acousto-opticaleffect of an ultrasonic beam can be also used.

Detecting means for detecting the laser beam which leaks out of thetransparent substrate 1 is provided above the transparent substrate 1.The detecting means has a CCD 6 and a lens system (image forming opticalsystem) 7 for forming, as an image onto the CCD 6, the light leaked outof the transparent substrate 1. An optical sensor for detecting thelight leaked out of the transparent substrata 1 is not limited to theCCD but a photo-multiplier or the like can be also used. When a flexiblelaser beam is used as illuminating light, the leak light from thesubstrate 1 is detected by eye observation and the detecting means canbe also omitted. When the CCD is used as detecting means, there are aCCD of a full frame system having a mechanical shutter function and oneof an interline system in which the mechanical shutter is not needed. Inconsideration of a durability, the CCD of the interline system ispreferable. In order to reduce noises, it is also desirable that the CCDhas a forced radiating fan or a thermoelectric cooling function.

An image processing apparatus 12 constructed by a computer or the liketo process a detected image is connected to the CCD 6 through an A/Dconverter 11 for converting a detected analog signal into a digitalsignal.

The image processing apparatus 12 has a function for analyzing an imagesignal from the CCD 6 and displaying a form pattern, a light quantity,an intensity distribution, or the like of the leak light due to theununiformity and a discriminating unit for discriminating the presenceor absence, kind (scratch or crack on the surface portion, striae,foreign matter in the inside, or the like), size (area, length, width,depth, region, or the like) of the ununiformity of the transparentsubstrate 1. As information (form pattern, light quantity, luminance,intensity distribution, depth from the surface, or the like of the leaklight) of the leak light which corresponds to the kind or size of theununiformity existing in the transparent substrate, measurement values(basic data) and the like previously obtained by measurements have beeninputted to a storage unit of the image processing apparatus 12.

A specific inspecting method executed by using the inspecting apparatusin FIG. 1 will now be described. As an inspecting object, a glasssubstrate for a photo mask, whose size is 152.4×152.4×6.35 mm and inwhich a width of the C plane is 0.4 mm is inspected. As shown in FIG. 2,the laser beam is impinged from the C plane of the glass substrate sothat an incident angle θi to the main surface on which the laser beamfirst impinges after it introduces through the glass substrate is largerthan a critical angle θc and an incident angle (90°−θi) to the endsurface of the glass substrate is larger than the critical angle θc.Since a refractive index of the glass substrate is 1.47 and the criticalangle θc is about 42.9°, the incident angle θi is set to 44.1°. That is,the method of introducing is characterized with respect to a point thatthe laser beam is introduced so that a singular point that the laserbeam (geometrically and optically) leaks does not exist on the mainsurfaces and end surfaces of the glass substrate and the introducedlaser beam is emitted only from the C planes. As a laser, an He—Ne laseris used and a laser beam in which a beam diameter is 0.5 mm, a spreadangle of the beam is 1 mrad, a laser power is 0.5 mW, and a wavelengthis 543 nm is irradiated.

Since the substrate which is used this time has a form in which all ofthe main surfaces as total reflective surfaces and the end surfaces asturning surfaces have such a relation as to cross at right angles toeach other, the incident angles of the light which impinges on the mainsurfaces are the same, the incident angles of the light which impingeson the end surfaces are the same, and the light propagates so that thoseangles have a predetermined relation (when the incident angle of thelight which impinges on the main surface is set to θi, the incidentangle of the light which impinges on the end surfaces is equal to90°−θi). Therefore, by merely setting so that the incident angle θi tothe main surface on which the light first impinges after it introducesthrough the glass substrate and the incident angle 90°−θi to the endsurface are larger than the critical angle θc, the light confinement isrealized. In case of a normal form (for example, a form in which themain surfaces and end surfaces do not have such a relation as to crossat right angles to each other, it becomes complicated slightly. In thisinstance, when it is assumed that the refractive index of the glasssubstrate for a wavelength λ of the laser beam which is introduced isset to nt, a refractive index ni of an external medium (air) which iscome into contact with the glass substrate is set to 1, an angle atwhich the light impinges on the main surfaces and end surfaces of theglass substrate is θik (k denotes a position where the laser beamimpinges on the main surfaces and end surfaces after it is introducedthrough the glass substrate and the incident positions k aresequentially set as k=1, 2, . . . ), the light confinement is notrealized unless the laser beam is introduced so that each θik is equalto the critical angle θ shown by sin θ=ni/nt or more. As mentionedabove, the case where the main surfaces and end surfaces have such arelation as to cross at right angles to each other is more effectiveagainst the light confinement.

As shown in FIG. 1, the laser beam introduced through the transparentsubstrate (glass substrate) 1 by the illuminating means repeats thetotal reflection on the main surfaces and end surfaces of the substrate1 and enters a state where the light is almost confined in the substrate1. The state where the light is almost confined means that theintroduced laser beam propagates in the transparent substrate andrepeats the total reflection and continues to propagate in thetransparent substrate until the light impinges on the chamfer portionserving as an introducing surface, namely, so long as it impinges on themain surfaces and end surfaces. Therefore, the laser beam incident inthe Y direction is scanned everywhere so as to be spread in a region(measuring region) of a cross section (YZ cross section) obtained bycutting the substrate 1 in the Y direction by the propagation due to thetotal reflection of the light itself.

As mentioned above, the laser beam introduced through the glasssubstrate repeats the total reflection on the main surfaces and endsurfaces of the glass substrate and enters the state in which the beamis almost confined in the glass substrate. However, when there is ascratch or the like on the glass surface due to a mixture of a foreignmatter upon polishing, the total reflecting conditions are notsatisfied, so that the light leaks out of a portion of the scratch. Withrespect to a defect of the glass in which a transmission is the same buta refractive index alone is different, which fact is peculiar to thestriae of the glass as well, the light is out of an inherent orbit(optical path) at a portion where the refractive index is different andleaks to the outside of the substrate 1 without being totally reflectedon the main surfaces and end surfaces. The leak light is detected by thedetecting means.

Due to the light irradiation in the above one cross section, theinspection of one line as observed from the main surface side can beexecuted. The inspecting process is executed by moving the table 5 inthe direction (X direction) of one side 1 a of the substrate 1 by thedriving means 4, so that the ununiformity of the whole region of thesubstrate 1 can be inspected. That is, it is a method of inspecting anununiformity in such a manner that in an inspecting region of asubstrate sandwiched by main surfaces as one certain pair of totalreflective surfaces and end surfaces as one certain pair of turningsurfaces of the substrate, an ununiformity (defect) on one certain planein the inspecting region filled with light by propagating the light inthe substrate is inspected and, after that, the inspected plane isrelatively moved in the direction which permits the inspecting region tobe filled with the light for the substrate.

A result detected by the inspecting apparatus is shown in FIGS. 4 and 5.FIG. 4 shows an image of a scratch on the surface of the glass substratedetected by the CCD 6. FIG. 5 is a graph obtained by performing an imageprocess to information of the light detected by the CCD on one certainside in the width direction of the scratch by a computer via an A/Dconverter (analog-digital converter). The CCD used at that time is a CCDof the interline system (having no mechanical shutter) in which athermoelectric cooling function is mounted and in which the number ofelements is 1300×1030, a detecting area is 8.71×6.90 mm, and asaturation amount of the CCD is 20000 electrons. As for the measuringconditions, an exposing time of the CCD is set to 200 msec.

An X axis of FIG. 5 denotes a coordinate in the width direction of thescratch. A Y axis denotes an intensity of the detected light. A unit ofa scale of the X axis is a pixel. In the inspecting apparatus, since anobjective lens of ×50 (50 times) and an image forming lens of ×0.45(0.45 times) are used, one pixel is 6.7 [μm]/(50×0.45), namely, itcorresponds to about 0.3 μm. (6.7 μm indicates a size of one pixel ofthe CCD.) The light intensity is resolved to 12 bits (4096:1) and onescale denotes (20000/4095)·Y(Y: scale) electrons. As will be obviouslyfrom FIG. 5, the intensity of the light leaked out of the scratch is(20000/4095)×4095=20000 electrons as a peak value that exceeds aallowance value of the CCD and the intensity of light in a region otherthan the scratch is equal to 0. As mentioned above, in an image which isdetected by the detecting means, the ununiform portion in which thescratch or the like exists is brightly seen in a linear or a dot form ina black background. The ununiform portion such as a scratch can beclearly discriminated from the obtained image. In this instance, whenthe propagation due to the total reflection in the glass substrate isconsidered, there is no factor that the light in the substrate is weakenduring the propagation except for a very slight absorption in theuniform portion, so that the light continues to propagate in the glasssubstrate. Therefore, almost all of the irradiated light concentrate onthe ununiform portion as a result. The ununiform portion sharply appearsat a very clear contrast. Therefore, a minute scratch or the like can bedetected at a high sensibility.

FIG. 6 shows an image of a scratch similar to that of FIG. 4 observed byan optical microscope (reflection·bright field). FIG. 7 is a graphshowing the image to which the image process similar to that in FIG. 5was performed. As will be understood from FIGS. 6 and 7, signals of thescratch are buried by signals of the background and the scratch cannotbe detected by the method. When the scratches of FIGS. 4 and 6 areobserved by an atomic force microscope (AFM), each of them is confirmedas a scratch in which the width is 0.13 μm and the depth is 0.0013 μm.

In the above embodiment, when it is desired to confirm the incidentangle θi of the laser beam to the transparent substrate 1, for example,as shown in FIG. 2, so long as an wedged optical member 8 is arranged onthe substrate surface via matching oil or the like, the incident angleθi can be obtained from a refractive index γ of light which is emittedfrom the optical member 8 or an apex angle of the optical member 8. Whensomething like the optical member 8 is used as an entrance window tointroduce the inspecting light into the substrate, the light can be alsointroduced from portions other than the chamfer portions (C planes) ofthe substrate.

FIG. 8 shows a relation between an S/N ratio and a normalized exposingtime when an arbitrary scratch is observed and the image process similarto the above is performed in order to clarify a difference between acase where the ununiformity is detected by the optical microscope in aconventional normal illumination and a case where the ununiformity isdetected by the inspecting method of the present invention. In thisinstance, the normalized exposing time is defined as (exposing time ofthe CCD)/(maximum exposing time of the CCD until the signals in thebackground reach (20000/4095)×100 electrons) and the S/N ratio is set to10·log₁₀(S/N). As will be obviously understood from FIG. 8, in theinspecting method in the conventional normal illumination, even when thenormalized exposing time is extended, an maximum is at most 3 dB. In theinspecting method of the present invention, the S/N ratio exceeds 30 dBas a result. In the inspecting method of the invention, the SIN ratio islimited up to 36 dB as maximum. It is because the saturation amount ofthe CCD camera is limited. It is considered that an extremely high S/Nratio exceeding 36 dB is actually obtained. (It is considered that thereason why the S/N ratio in the normal illumination indicates a minusvalue is that the signal of the scratch is buried by noises.) Accordingto the inspecting method of the present invention, therefore, the S/Nratio exceeds 4.8 dB (the signal for the background is equal to anamount that is three times as much as that of noises) which is generallyknown as an image processing possible level by far, so that the presenceor absence, kind, and size of the ununiformity in the transparentmaterial can be accurately discriminated.

In the above embodiment, the incident angle θi is set to 44.1°. Theoptimum incident angle at which the total reflection is repeated morecan be easily selected by simulations shown as follows. The resultsobtained by simulating a situation in which the light propagates in thetransparent substrate will now be explained hereinbelow.

First, a calculation result when the light is propagated along one side(in the y axial direction) of the transparent substrate 1 as shown inFIG. 9 will now be described. In the simulations, the dimension of thetransparent substrate 1 is 152.4×152.4×6.35 mm that is the same as thatof the glass substrate for the photo mask in the foregoing embodiment.The width of C plane is set to 0.4 mm. The refractive index of thetransparent substrate 1 is set to 1.47 that is a refractive index ofquartz glass and the refractive index around the transparent substrate 1is set to 1.00 that is a refractive index of air. A vector (unit vector)indicative of the direction of the beam which impinges on the C plane(which forms an angle of 45° against the main surface and T plane)obtained by chamfering the transparent substrate 1 is set to (0.0000000,0.6864532, −0.7271740). FIG. 10 shows results of the simulations.

In FIG. 10, the incident angle is the incident angle to the surface(main surface) on which the beam first impinges after it enters thesubstrate 1. The incident angle is changed every 0.05 degree of angle. Az coordinate upon emitting indicates a z coordinates when the beam isemitted from the transparent substrate 1 and the bottom surface of thesubstrate 1 is set to z=0.

FIG. 11 is a graph showing the number of reflecting times on thesurfaces at respective incident angles. As will be understood from FIG.11, it is sufficient that the incident angle at which the number ofreflecting times on the surfaces increases is selected in accordancewith the form or the like of the transparent substrate. It is alsosufficient that the incident angle of the light which is introduced isfluctuated.

FIG. 12 shows states of the propagation of the light in the substrate incase of the incident angle of 43.35°. FIGS. 12(1), (2), and (3) showstates in which the number of reflecting times on the surfaces are setto 50, 250, and 661 times (upon emitting), respectively. In thissimulation, in order to simplify the calculation, the propagation isperformed in the only region in the cross section of one plane (yzplane). As shown in FIG. 12, it will be understood that the light beamrepeats the total reflection and propagates so as to fill the region. Ina manner similar to the simulation, when the parallel light isintroduced in the one side (y direction) of the transparent substrate,in order to impinge the illuminating light on the whole region of thetransparent substrate, it is sufficient that the light is scanned alonganother side (x direction) by using a mirror or the like or the lightspread in a slit shape in the x direction is introduced from the Cplane.

The simulation results when the width of C plane, refractive index ofthe glass substrate (corresponding to the wavelength of the laser beam)in the simulation of FIG. 9 are changed are shown in FIGS. 13, 14, and15. The simulation is executed under the conditions similar to those ofthe simulation executed in FIG. 9 other than a fact that the refractiveindex of the glass substrate is set to 1.46 (corresponding to thewavelength of the laser beam that is equal to 543 nm) and the width of Cplane is changed to 0.2 mm (FIG. 18), 0.4 mm (FIG. 14), and 0.8 mm (FIG.15).

As will be understood from FIGS. 13 to 15, as the width of C planeincreases, the number of reflecting times on the surfaces decreases. Thereason is as follows. When the laser beam introduced from the C planepropagates in the transparent substrate and again impinges on the Cplane, the beam leaks without being totally reflected because the lightincident to the C plane is impinged at an angle that is smaller than thecritical angle θ. Consequently, a probability that the light propagatedin the transparent substrate impinges on the C plane is raised byincreasing the width of C plane. Therefore, in order to increase thenumber of reflecting times on the surfaces in the transparent substrate,it is sufficient to reduce the width of C plane. In case of the glasssubstrate (152.4×152.4×6.35 mm) for the photo mask used at this time,since the transparent substrate is sufficiently filled with the light solong as the number of reflecting times on the surfaces is about 300times, it is preferable that the width of C plane is equal to 0.4 mm orless.

When FIGS. 11 and 14 in each of which the width of C plane is 0.4 mm arecompared, the critical angle for satisfying the total reflectingconditions is changed by changing the refractive index (or wavelength ofthe laser beam (because the refractive index of the transparentsubstrate is decided by the wavelength of the laser beam)) of thetransparent substrate, so that the number of reflecting times on thesurfaces can be adjusted. As for the critical angle to satisfy the totalreflecting conditions, as a difference between the refractive index ofthe transparent substrate and that of the external medium (for example,the air) of the transparent substrate is larger, a degree of freedom ofthe critical angle increases. In accordance with the above, the numberof reflecting times on the surfaces also increases. In fact, however,there is a case where the material of the transparent substrate islimited depending on the use. Therefore, ordinarily, the number ofreflecting times on the surfaces can be adjusted by properly selectingthe wavelength of the laser beam. As a wavelength of the laser beam, thewavelength in which the absorption for the transparent substrate islittle is preferable. Since it is influenced on the resolution of theununiformity, the wavelength of the laser beam is selected inconsideration of the following points.

FIG. 16 shows loci of the light beam in the general direction introducedthrough the transparent substrate, which is not parallel to the one sidesimilar to the above simulation. In the simulation, a vector indicativeof the direction of the light beam which impinges on the C plane of thetransparent substrate 1 is set to (0.6924, 0.3823, −0.6117) and theconditions other than the above are the same as those of the simulation(FIG. 9). As shown in the diagram, the introduced light beam repeats thetotal reflection in the transparent substrate 1 and is substantiallyconfined in the substrate, so that the light propagates in the wholearea of the substrate. Therefore, even when the scanning of theilluminating light is not executed at all, the whole range of thetransparent substrate as an inspecting region can be inspected in a lumpat a high speed.

In consideration of the simplification of the inspecting method, thecase where one certain surface of the transparent material is decided asshown in the above embodiment, the incident angle satisfying the totalreflecting conditions is determined in the surface, the light isintroduced, and after that, the incident position of the light is movedin accordance with the form of the transparent material is morepreferable than the case where three dimensional directional vectors (x,y, z) of the incident light are set so that the introduced light coversthe whole region in the transparent material and the light is introducedfrom a certain point on the substrate, because the inspecting method canbe simplified. When the transparent material is the substrate having thesurfaces which face each other, it is particularly effective.

In the above embodiment, the example in which the laser beam wasintroduced from one side 1 a of the transparent substrate 1 has beenmentioned. The invention is not limited to the above but it is alsosufficient that the inspection is executed by introducing the light fromthe direction of one side 1 b or from two directions of the sides 1 aand 1 b. When the inspection is executed by introducing the light fromthe two directions of the sides 1 a and 1 b, it is effective to detect adefect having directionality or the like and the inspection at a higherprecision can be performed, so that it is preferable.

As mentioned above, the present invention is extremely effective withrespect to the detection of the defect having directionality for thelight, which can be detected because it glints in a specific irradiatingdirection but cannot be detected because it does not glint in the otherirradiating directions, which fact is peculiar to a scratch on glass.The reason is as follows. Since the light is substantially confined inthe inspecting object made of the transparent material by geometricallyand optically repeating the total reflection, the irradiated light isdeviated from an inherent orbit only in the ununiform portion of theinspecting object and leaks out of the inspecting object from thegeometrical and optical viewpoints. Even if the only ununiform portionis the defect having directionality for the light, the ununiform portionis illuminated from various directions in the process to repeating thetotal reflection. In the conventional method, since the light isconverged in order to raise the contrast and the light from the onedirection is irradiated, even if the defect has a relatively large size,the defect having the directionality can be hardly detected.

Also with regard to the defect of the glass in which the transmittanceis the same but the refractive index alone is different, which fact ispeculiar to the striae of the glass, the light is deviated from theinherent orbit in a portion where the refractive index is different andleaks out of the inspecting object, so that the defect can be detected.In the conventional method of detecting a light amount such asreflection output or transmission output of the converged light,however, the detection is impossible in principle.

By using the inspecting method of the above-mentioned embodiment, theglass substrate having the defect can be rapidly and properly excluded,so that the productivity of the glass substrate can be improved. Byagain precisely mirror-polishing and cleaning the glass substrate havingthe defect such as a scratch on the surface, it can be formed as a glasssubstrate for the photo mask in a range of a specification.

The above inspecting method is used in the inspecting process after themanufacturing process of the glass substrate as a transparent substratefor the photo mask. There is a variation in size (length or the like) ofthe glass substrate depending on a difference of the processingprecision (ordinarily, a allowance of the transparent substrate for thephoto mask has about ±0.4 mm in length and about ±0.1 mm in thickness).Therefore, when variant sizes of the glass substrates are grasped one byone, the optimum total reflecting conditions for each of the glasssubstrates are obtained, and they are inspected, it takes a long timeand it is not practical. Because, when such an inspecting method thatthe accurate dimensions of the glass substrates are measured, theincident conditions under which the total reflection more occurs aregrasped, and after that the laser beam is impinged, an extra time asmuch as {(time for measuring the dimensions of the glasssubstrates)+(simulating time)}×(the number of inspecting substrates) isrequired before the inspection.

In this case, by fluctuating the incident angle of the laser beam whichis introduced through the glass substrate in a range where the totalreflection is done on the main surfaces (surface and rear surface) andend surfaces (other than the C planes) and the light is reflectedbetween at least the pair of end surfaces and impinging the laser beam,even when there is a variation in dimensions of the glass substrates,the optical ununiformity of the glass substrate can be detected at ahigh sensitivity and a high speed, so that there are providedununiformity inspecting method and apparatus of a high utility.

That is, when the optical path in the transparent material is opticallyuniform, the incident angle is changed in a range where the totalreflection may occur on the surface of the transparent material and thelight is introduced in the transparent material. Consequently, even whenthere is a variation in dimensions of the transparent materials and theoptimum total reflecting conditions for the transparent materialsslightly differ, the incident light of a predetermined direction is notintroduced but the light having different incident angles is introducedand propagates various paths while being totally reflected, so that thelight is spread up to the corners of the transparent material withoutleaking.

As fluctuating the incident angle for the substrate, in a manner similarto the angle adjusting means in FIG. 1, a machine which is connected toa computer or the like and which can automatically control the angle isattached to the mirror, an angle adjusting mechanism is provided for thelaser itself or the folder for holding the substrate, or means such asan acousto-optical polariscope for fluctuating the incident angle byusing the acousto-optical effect of the ultrasonic beam can be alsoused. As an incident angle to the substrate, for example, in case of theabove-mentioned glass substrate (152.4×152.4×6.35 mm) for the photo maskmade of quartz glass, it is desirable that the incident angle θi of thelaser beam is successively changed in a range of 45.0° to 44.0°.

The introduction of the laser beam to the transparent material isperformed on the basis of information of the transparent material.Consequently, when a plurality of transparent materials are inspected,particularly, the inspection can be efficiently executed.

In this instance, the information of the transparent material indicatesthe relative positional relation between the transparent material andilluminating means, the state of the surface of the transparent material(whether it has been mirror-polished), or the like. The informationregarding the relative positional relation between the transparentmaterial and the illuminating means is necessary to properly introducethe light from the illuminating means into a predetermined position ofthe transparent material. When the surface is not in the mirror state,it is difficult to detect the ununiformity of the transparent material.Therefore, the information regarding the surface state of thetransparent material can be used when such a substrate is previouslyeliminated (as necessary, it is returned to the preceding process(polishing or the like)).

By providing position detecting means for detecting the relativepositional relation between the transparent material and theilluminating means and transmitting means for transmitting informationobtained by the position detecting means to the illuminating means, whena plurality of materials are inspected, the inspection can beefficiently executed. In this instance, the position detecting meansindicates a distance measuring device (laser scan measuring system,laser interference measuring device, or the like) by using the laserbeam. The transmitting means indicates a computer for fetching data fromthe position detecting means and feeding back to the illuminating means,the angle adjusting means, moving means for moving the incident positionof the introduction light, and the like. It is also sufficient toprovide an apparatus such as TV camera or CCD image pickup device imagesensor for observing the surface state of the transparent material and,for example, eliminating the inspecting object, for example, which isnot mirror-polished.

In the above embodiment, it is desirable to provide discriminating meansfor discriminating the presence or absence, kind, and size of theununiformity of the transparent material on the basis of the informationof the light detected by the detecting means.

The relation (information) between the presence or absence, kind(scratch or crack of the surface portion, internal striae or foreignmatter), and size (area, length, width, depth, region, or the like) ofthe ununiformity previously existing in the transparent material and theinformation of the light which leaks out of the surface (light amount,luminance, intensity distribution, depth from the surface of the leaklight) has been stored in the computer or the like. By comparing theinformation of the light detected by the inspection and the storedinformation, the presence or absence, kind, and size of the ununiformityof the transparent material can be discriminated. By discriminating thepresence or absence, kind, and size of the ununiformity of thetransparent material, a desired transparent material can be immediatelyextracted. Consequently, the glass substrate having the ununiformitywhich has an influence on the time of formation of the pattern or theexposure for a transferring object can be eliminated before the nextprocess after the inspection or can be also returned to the re-polishingprocess, so that the productivity can be improved.

The discriminating method will now be specifically explained by usingthe inspecting apparatus in FIG. 1. The light leaked out of thesubstrate 1 is formed as an image on the surface of the CCD 6 of the CCDcamera by the lens system 7. As mentioned above, for a period of timewhile the laser irradiating region of one line is scanned onto the wholesurface of the main surface of the substrate 1, the shutter of the CCDcamera is opened as It is and image data of the whole surface of themain surface of the substrate 1 is accumulated. The image data fetchedin the CCD camera is converted to a digital signal by the A/D converter11, the converted signal is inputted to the image processing apparatus12 and is stored into the storage unit, and an image analysis isperformed by the discriminating unit. In the discriminating unit, bycomparing the image data of the light detected by the inspection withbasic data of the image information which has previously been inputtedin the storage unit, the presence or absence, kind, and size of thetransparent substrate 1 are discriminated. The moving amount(information of the irradiating position of the substrate 1) of thetable 5 or the like is inputted from a laser interferometer (not shown)or the like to the image processing apparatus 12. From the image data ofthe CCD camera and position data of the substrate 1, which kind and sizeof ununiform portion exists on which position (x, y) of the substrate 1is obtained.

When the ununiform portion exists in the irradiating region of thesubstrate 1, the ununiform portion (and its periphery) is brightly seenin a dot form. When they are enlarged by the optical microscope, imagesas shown in FIG. 17 are observed (the images in FIG. 17 are shown byreversing bright and dark of images that are actually observed). Alinear image 41 as shown in FIG. 17(a) is a scratch on the surface ofthe substrate 1. As a representative size, the length is 30 μm, thewidth is 0.2 μm, and the depth is 0.002 μm (the size of such a finescratch was measured by the atomic force microscope). Many collectedimages 42 as shown in FIG. 17(b) are caused by striae or foreign matterssuch as gasses in the substrate 1. As a representative size, thediameter is about 1 mm. As mentioned above, since the pattern or size ofthe image differs depending on the ununiformity existing in thesubstrate 1, the kind of ununiformity can be discriminated. Further,since the images 42 by the striae are seen so as to glimmer as comparedwith the image 41 by the scratch, it can be also discriminated from theluminance or intensity distribution of the image.

The size of the ununiform portion can be discriminated from a lightamount of the detected light. Further, whether the position where theununiformity exists is located on the surface portion of the substrate 1(scratch or clack) or in the substrate 1 (striae or foreign matter) canbe discriminated from a location (depth) where a focal point is obtainedby focusing on bright dotty portions of the substrate 1 by the opticalmicroscope. As for the inspection of the ununiformity, in order torealize a high speed process, it is desirable that whether the brightdotty leak light exists on the surface of the substrate 1 is firstinspected and, with respect to only the substrate 1 in which the leaklight was detected, bright dotty portions is further inspected byenlarging or the like by the optical microscope.

In order to discriminate the ununiformity, when the light is introducedfrom the different incident positions and different directions (twodirections) to the substrate 1, accurate information of the leak lightcan be obtained in even case of the ununiformity (defect) havingdirectionality. Consequently, since the presence or absence, kind, andsize of the ununiformity can be accurately discriminated, it ispreferable. As a method of introducing the light, as shown in FIG. 18, alaser beam L1 is introduced in the direction (X direction) of the side 1a of the transparent substrate 1 and a laser beam L2 is introduced inthe direction (Y direction) of the side 1 b at the same time or it isalso sufficient that the laser beams of the different directions areintroduced every direction (X and Y directions or the like) to thesubstrate 1, thereby inspecting.

As shown in FIG. 19, when the ununiformity is inspected by introducing alaser beam L from the corner portion of the transparent substrate 1 byusing, for example, a laser 13 and mirrors 14 and 15, the kind and sizeof the ununiformity can be discriminated by an image forming opticalsystem 16, a CCD camera 17, and an image processing apparatus 18 in amanner similar to the above.

In the above embodiment, although the example in which the inspectionwas performed by introducing the laser beam in only the y direction ofthe substrate 1 has been mentioned, when the laser beam is introducedfrom the different incident positions and different directions to thesubstrate 1, it is not necessarily that the incident positions are madedifferent. For example, as shown in FIG. 20, even when the laser beamsL1 and L2 are introduced from the same incident position of thesubstrate 1 to a plurality of different directions as observed from themain surface side of the substrate 1, the effect of the invention thatthe ununiformity having directionality can be certainly detected can beaccomplished.

Although the introduction of the laser beam to the transparent substratehas been performed from the chamfer portion as a C plane in the aboveembodiment, it is also possible to introduce the beam from planes otherthan the chamfer portions. In this case, it is sufficient that as anentrance window to introduce the laser beam, an optical member made of amaterial having substantially the same refractive index as that of thetransparent substrate is attached by an adhesive agent or the like. Inorder to simplify the inspecting method and inspect the ununiformity ofthe whole region in the transparent substrate by repeating the totalreflection more, it is desirable to introduce the laser beam from thechamfer portion as a C plane. The reason is that when the optical memberhaving the entrance window to introduce the laser beam is attached, thelight propagated in the transparent substrate does not satisfy the totalreflecting conditions in a portion of the optical member, so that thelight leaks out of the portion. It is preferable that the chamferportion is mirror-polished. As the width of chamfer portion is smaller,it is more desirable. It is better to set the width to be equal to 0.4mm or less, more preferably, 0.2 mm or less. Even if it is set to beextremely small (smaller than 0.1 mm), since a defect occurs uponmirror-polishing, it is not preferable.

A method of selecting the transparent substrate which can be used forvarious uses by using the inspecting method and inspecting apparatus ofthe invention will now be explained with reference to the drawings. FIG.21 is a diagram showing a flowchart for the inspecting process toselecting the transparent substrate.

A specific selecting method performed by using the inspecting apparatusin FIG. 1 will now be described with reference to the flowchart for theinspecting process of FIG. 21.

Decision·Alignment of the Inspecting Region

As a transparent substrate 1 serving as an inspecting target, a glasssubstrate for a photo mask made of quartz glass in which both mainsurfaces, end surfaces, and chamfer surfaces are mirror-polished, whosesize is 152.4×152.4×6.35 mm, and in which the width of each C plane isequal to 0.4 mm is prepared.

The glass substrate is conveyed by conveying means (not shown) until itis come into contact with a stage guide pin (not shown) fixed to acertain reference position of the inspecting apparatus, therebypositioning the glass substrate (process 1). At that time, an origin andcoordinates in the glass substrate are decided.

The inspecting region is specified on the basis of the previouslydecided coordinates. The inspecting region does not always coincide witha measuring visual field of the CCD. Therefore, when they don't coincidewith each other, the measuring region is divided into (A1, A2, A3, B1,B2, B3, . . . ) in correspondence to the visual field of the CCD (FIG.22) (process 2). In this instance, the divided measuring regions A1, A2,A3, B1, B2, B3, . . . coincide with the measuring visual field of theCCD. The CCD used for the measurement is the CCD of the interline system(having no mechanical shutter) having a thermoelectric cooling functionis mounted, in which the number of elements is 1300×1035 and a detectingarea is 8.71×6.90 mm. The measuring visual field is measured at amagnification of 0.7 time.

The incident position and the incident angle of the laser beam areadjusted so that the laser beam propagates in the inspecting area(process 3). As for the incident position and incident angle of thelaser beam, information of the glass substrate is obtained by positiondetecting means (not shown) for detecting the relative positionalrelation between the transparent substrate and the laser and the laserbeam is introduced by adjusting the mirrors and table so that the laserbeam can be accurately introduced through the glass substrates havingdifferent sizes. As for the incident angle, since the refractive indexof the glass substrate is 1.46 and the critical angle θc is about 43.2°,the incident angle θi is set to 45.0°.

When the laser beam is introduced into the glass substrate, the incidentangle of the laser beam is fluctuated in a range where the beam repeatsthe total reflection and propagates (process 4). In the procedure forinspecting a plurality of glass substrates, even if the respective glasssubstrates have a little variation in sizes depending on a differencebetween the processing precisions, since the beam locus propagating inthe glass substrate is deviated little by little by changing theincident angle of the light, the process is executed in order to absorbthe variation of processing precisions of the glass substrate andinspect the ununiformity of the glass substrate. The process is alsoexecuted for an image matching of the CCD in the next process. As meansfor fluctuating the incident angle, means having a function forautomatically adjusting the angle of the mirror by a control of acomputer or the like or means such as an acousto-optical polariscope forfluctuating the incident angle by using the acousto-optical effect ofthe ultrasonic beam can be also used. As a fluctuation of the incidentangle, the incident angle θi is successively changed in a range of 45.0°to 44.0° so as to satisfy the total reflection.

In order to precisely discriminate the light which leaks out of thetransparent substrate, namely, the ununiformity, the focusing of the CCDimage is executed (process 5). The focusing is executed by integratedlymoving the laser and the mirrors in an a axial direction (direction tothe lens and CCD). It is also sufficient that the glass substrate,mirrors, and laser are fixed and the lens and CCD are integratedly movedin a z axial direction.

Inspection of Ununiformity in Inspecting Region

As shown by enlarging one part in FIG. 22, the laser beam L isintroduced from the chamfer plane serving as an introducing surface sothat the laser beam L which passes certain coordinates (A1X1, A1Y1) ofthe measuring region A1 as one of the divided regions and is parallel tothe y axial direction propagates, the incident angle is changed in arange where the laser beam repeats the total reflection and propagatesin the glass substrate (of 45.0° to 44.0°), thereby inspecting theununiformity. The similar scan is performed so that the laser beam L ismoved in the x axial direction, the inspection of the ununiformity isperformed until the laser beam passes coordinates (A1X1, A1Y1) at theend portion of the measuring region A1, so that the inspection of theununiformity in the measuring region A1 is finished (process 6). Theexposure of the CCD is executed from the start to the completion of theinspection of the ununiformity in the measuring region A1. As for theinspection of the ununiformity in the measuring region A1, it is alsosufficient that the laser beam is impinged so that the laser beam whichpasses (A1X1, A1Y1) and is parallel to the x axial direction propagatesand it is moved in the y axial direction. It is also sufficient tocombine and introduce the laser beams in the two directions which crossat right angles to each other. As mentioned above, in case ofintroducing the light of a plurality of different directions as observedfrom the main surface side, the light is irradiated into the glasssubstrate from the plurality of directions. Even when the ununiformity(defect) having directionality exists, it can be accurately detected.

Image Process

In the process 6, information (analog signal) of the light which wasdetected by the CCD and which leaked out of the glass substrate isconverted into a digital signal by the A/D converter in order to executean image process by information accumulating means such as a computer.The information of the light converted into the digital signal isaccumulated by the information accumulating means such as a computer,the intensity of the light as shown in FIG. 28 is resolved to 12 bits(4096:1), and the image process is performed (process 7). The Y axis inFIG. 23 denotes the intensity of the light and one scale denotes(20000/4095)·Y(Y:scale) electrons.

Discrimination of Allowance of Ununiformity

As a result of the image process of the process 7, the ununiformityexisting in the glass substrate is discriminated as a scratch on thesubstrate surface. When it is compared with (20000/4095)×200 electronswhich has previously been set as an allowance design value of thescratch (when the background is (20000/4095)×100 electrons or less),since it exceeds the allowance design value, it is discriminated thatthe glass substrate is bad (process 8).

In the embodiment, since the scratch exceeding the allowance range isfound on the substrate surface in the inspecting area A1, the inspectionfor the ununiformity is not executed in the next inspecting area A2 butthe process is shifted to the re-polishing and cleaning process for thesubstrate. When no ununiformity is found in the inspecting region A1,the previously divided inspecting regions A2, A3, B1, B2, . . . and theprocesses 6 to 8 (according to circumstances, processes 2 to 8) arerepetitively executed. When it is discriminated that the ununiformity isequal to the allowance design value or lower in the whole inspectingregion, the glass substrate for the photo mask is selected as good.

By using the selecting method of the embodiment, the glass substratehaving the defect can be rapidly and properly eliminated, so that theproducibility of the glass substrate can be improved. By again preciselymirror-polishing and cleaning the glass substrate having the defect, itcan be formed as a glass substrate for the photo mask which lies in therange of the specification.

FIG. 24 is the second embodiment in which the method of selecting thetransparent substrate of the invention is applied to a glass substratefor a magnetic disk. The explanation for the processes overlapped in thefirst embodiment in which the method is applied to the glass substratefor the photo mask is omitted.

As a transparent substrate 1 serving as an inspecting target, adisc-shaped glass substrate for magnetic disk made of quartz glass inwhich both main surfaces (H), inner rim edge surface (T1 plane) andouter rim edge surface (T2 plane), and chamfer surfaces (C planes) aremirror-polished and in which the diameter is 95 mm (3.5 inchesφ), thethickness is 0.8 mm, and a diameter of a circular hole on a centerportion is 20 mm φ is prepared.

As inspecting regions, as shown in FIG. 24, the region on the mainsurface of the transparent substrate is divided into the measuringregions A1, A2, A3, . . . from the inner rim side to the outer rim side.The inspection regarding the ununiformity is executed every dividedmeasuring region.

The inspection for the ununiformity is executed in such a manner thatthe laser beam L is introduced from the outer rim edge surface-of thedisc-shaped glass substrate 1 in the direction of the center (O) of thedisc, the light is confined in one plane in the radial direction (rdirection) including the outer rim edge surface and the inner rim edgesurface (so that the total reflection is repeated in both the mainsurfaces of the transparent substrate and the light is returned betweenthe inner rim and outer rim edge surfaces), the disc is rotated by adriving apparatus (not shown) for rotating the disc, and the laser beamL is moved in the direction to the rim (θ direction) of the disc.Specifically explaining with reference to FIG. 25, the laser beam L isintroduced to the chamfer surface (C plane) as an introducing surface bya laser 25 and mirrors 26 and 27 so that the laser beam L which passescertain coordinates (A1r1, A1θ1) of the measuring region A1 divided asshown in FIG. 25 and is parallel to the r direction is propagated, theincident angle is changed in a range (45.0° to 44.0°) where the beamrepeats the total reflection and propagates in the disc-shaped glasssubstrate 1, thereby inspecting the ununiformity. The disc-shaped glasssubstrate 1 is rotated, the same scan is performed by moving the laserbeam in the θ direction, and when the inspection for the ununiformity inthe region which passes the coordinates (A1r1, A1θX) of the measuringregion A1 is finished, the inspection of the ununiformity in themeasuring region A1 is completed. In the inspection for theununiformity, it is also sufficient that the laser beam is impinged fromthe inner rim edge surface of the disc or from both of the inner rim andouter rim edge surfaces.

In a manner similar to the first embodiment, the image process andallowance discrimination for the ununiformity are executed.Consequently, since the defect exceeding the allowance range is notfound in the inspecting region A1, the inspecting region is changed tothe inspecting regions A2, A3, B1, B2, . . . and the inspection for theununiformity similar to that in the inspecting region A1 is executed.Although the inspection for the ununiformity is performed in the wholeregion of the disc-shaped glass substrate, no defect exceeding theallowance range is found, so that it is discriminated as good.

In a manner similar to the first embodiment, when the defect exceedingthe allowance range is found in the certain inspecting region, it isdiscriminated as bad and the process can be also shifted to there-polishing and cleaning processes for the substrate without executingthe inspection for the ununiformity in the following inspecting region.

In the above-mentioned inspecting method and selecting method, asunnecessary light which leaks out of the ununiformity (defect) of thetransparent material and which allows the contrast of the light to bedecreased, Rayleigh scattering light which scatters due to a microscopicfluctuation of a density that is peculiar to the transparent material orthe like exists. In order to reduce the unnecessary light, at least twolight having different wavelengths are introduced into the transparentmaterial or light having a specific polarization is introduced, so thatthe contrast of the detection light for the ununiformity is improved andthe detection can be realized at a higher sensitivity and a higherprecision. In the former case of introducing at least the two lighthaving different wavelengths, since the Rayleigh scattering lightbecomes light having a color obtained by mixing the light havingdifferent wavelengths, the scattering light can be eliminated byapplying a (color) filter for absorbing or reflecting a wavelength areaof the mixed light between the transparent material and the detectingmeans. In the latter case of introducing the light having the specificpolarization, the light becomes light having peculiar polarizingcharacteristics in a peculiar polarizing state by the Rayleighscattering. By using a difference between the polarizing characteristicsand those of light which leaks by the ununiformity, a polarizing devicesuch as polar screen, polarizing plate, or polarizing prism is placedbetween the transparent material and the detecting means, so that theRayleigh scattering light can be effectively eliminated.

As unnecessary light which decreases the contrast of the light thatleaks out of the ununiformity (defect) of the transparent material,there is stray light derived in such a manner that light which was notintroduced in the transparent material is reflected on the surface ofthe transparent material and is impinged on the detecting system fordetecting detection light for the ununiformity. In order to reduce thestray light, the introduction light is reduced in correspondence to thesize of the introducing surface of the transparent material on which thelaser beam is impinged by converging the beam by the optical system suchas a lens and the introducing surface is shaped into a concave crosssection so that the converged laser beam is introduced as almostparallel light into the transparent material from the introducingsurface. Consequently, the stray light can be reduced, the contrast ofthe detection light for the ununiformity can be increased, and thedetection can be executed at a high sensitivity and a high precision.

As another factor to reduce the contrast of the light which leaks out ofthe ununiformity (defect) of the transparent material, as shown in FIG.26 (in the diagram, (1) is a perspective view and (2) is a crosssectional view), there is a case where the light shielding material is arectangular plate having main surfaces, end surfaces, and chamfersurfaces, when the ununiformity is inspected by introducing the laserbeam L from the introducing surface (chamfer surface (C plane)), thelight leaks out of the chamfer surfaces other than the introducingsurface and becomes stray light. In this case, portions between thechamfer surfaces other than the chamfer surface which face each other inthe progressing direction of the light to introduce the light areconnected by an introducing light 50 formed by binding a plurality ofoptical fibers arranged in the surface direction of the chamfersurfaces, thereby enabling the light which leaks out of the chamfersurfaces to be again introduced through the transparent material.Therefore, the stray light is reduced, the introduced laser beam can bemore effectively concentrated to the ununiform portion, so that thecontrast is increased and the detection can be realized at a highsensitivity and a high speed.

In the embodiment in the inspecting method and the first and secondembodiments in the selecting method, the transparent substrate made ofglass has been mentioned as a transparent material having themirror-polished surfaces. It is not limited to glass but any material,for example, optical plastic such as acrylic resin or optical crystalsuch as quartz through which the inspection light can transmit can beused.

In the embodiment in the inspecting method and the first and secondembodiments in the selecting method, the example in which the wholesurface of the transparent substrate was mirror-polished has beenmentioned. It is not limited to the above but a transparent substratehaving a part or whole of the surface which is not mirror-polished canbe also used. For example, in case of the glass substrate as a glasssubstrate for the photo mask, there is a case where the end surfacesother than the main surfaces, in which the pattern is not formed, arenot mirror-polished. In case of the glass substrate for the magneticdisk, there is a case where the inner rim and outer rim edge surfaces inwhich a film such as a magnetic layer is not formed are notmirror-finished. In this case, by coating a liquid such as a matchingoil onto the surfaces which are not mirror-finished, the surfaces lookas if they are mirror-polished (free surface of liquid, pseudo mirrorsurface), so that the ununiform portion can be inspected by theinspecting method and inspecting apparatus of the invention. Moreparticularly, when it is desired that the ununiform portion alone(striae, bubbles, foreign matter, or the like) existing in thetransparent material is inspected at a stage where the mirror-finishingis not executed, it is effective.

As a liquid which is coated in order to form the pseudo mirror surface,a matching oil or a sealing agent used for optical parts, or maskingagent for scratch of the glass can be mentioned. The liquid coated onthe surface of the transparent substrate can be in a liquid state as itis or in a solid state of jelly, a hard film, or the like aftercompletion of the coating. As a coating method of the liquid, any methodsuch as brush coating (a brush or a sponge-like material is soaked inliquid, thereby coating), spray coating, or spin coating which cansmoothly coat on the surface of the transparent material can be used. Inthis case, the proper method is selected in accordance with the liquidwhich is used or coating surface.

When the refractive index of the transparent material is substantiallythe same as that of the liquid, the liquid-coated surface in a mirrorstate optically and substantially becomes the surface of the transparentmaterial, so that the light introduced through the transparent materialcan be certainly totally reflected and be returned into the inside.Specifically speaking, since quartz glass (refractive index is 1.46) orthe like is often used as a transparent substrate, as a liquid whoserefractive index is approximate to the above and which can be easilyhandled, canada balsam (refractive index is 1.52), Enterannew (tradename, refractive index is 1.49), diiodemethane (ethylene iodide,refractive index is 1.74), cedar oil (refractive index is 1.52), liquidparaffin (refractive index is 1.48), Aquatex (trade name, refractiveindex is 1.4), glycerol (refractive index is 1.46), and the like can bementioned.

As for a water insoluble material such as canada balsam or Enterannew,the refractive index and viscosity can be adjusted by adding organicsolution such as xylene. As for a water soluble material such asglycerol or Aquatex, the refractive index and viscosity can be adjustedby adding water. As a masking agent for scratch of the glass, there isemulsion composition in which polyorganosiloxane andpolydiorganosiloxane are main components disclosed in Japanese PatentApplication Laid-Open Publication No. 6-4496 (1994) or the like.

As an inspection in the case where the whole surface of the transparentsubstrate is not mirror-finished, for example, there is a case where theununiformity alone (striae, bubbles, foreign matter, or the like) in thetransparent substrate is inspected. In this case, when the ununiformportion exists In the inside, it results in fatal defect. For example,in case of the glass substrate for a phase shift mask, since the bad onecan be eliminated by inspecting at a stage before the mirror-finishing,the costs of manufacturing can be also held down.

The form of the transparent material is not limited to the square(rectangular) or circular substrate but the transparent material cantake any form of a block form, a sphere, a column, a cylinder, apolyhedron, and a form having curved surfaces. Particularly, when asubstrate having surfaces which face each other, more particularly, asubstrate having at least two pairs of parallel planes which face eachother (for example, square (rectangular) or circular cone) is used assuch an above-mentioned transparent material, the introduced lightrepeats the total reflection and easily enters a state in which thelight is confined in the substrate. In fact, the inspection for the wideregion of the transparent material can be simultaneously executed andthe inspection can be executed at a high speed. As a substrate, further,the inspection can be applied to various kinds of substrates such asglass substrate for the electronic device (for the photo mask (phaseshift mask)), glass substrate for the liquid crystal display, or glasssubstrate for information recording (such as magnetic disk or opticaldisk) Since the glass substrate for information recording isdisc-shaped, in case of actually inspecting, the laser beam is impingedfrom the polished outer rim or inner rim edge surface (for example,chamfer portion). When the inspection for both the surfaces of thesubstrate is needed, it is also sufficient that the detecting means areprovided on the upper and lower sides of the substrate, respectively,and both of the substrate surfaces are inspected in a lump.

In the above embodiment, the gas laser (He—Ne laser) has been used as alaser. It is not limited to the above but a laser of a visible area suchas a semiconductor laser or, so long as it is absorbed to thetransparent substrate by little, an excimer laser of an ultravioletarea, an Nd-YAG laser of an infrared area, a CO₂ laser, or the like canbe used as a light source for the inspection. Particularly, in case ofusing the laser of the ultraviolet area (for example, a higher harmonicwave of the excimer laser, a YAG laser, or the like), since the foreignmatter or the like adhered on the substrate surface can be eliminateddue to the operation such as evaporating or transpiring, it ispreferable.

In the above embodiment, the example in which the angle adjusting meansfor changing the incident angle for the substrate was attached to themirrors located between the laser and the substrate has been mentioned.So long as the incident angle of the laser beam for the substrate can bechanged, any construction can be used. It is also sufficient that theangle adjusting means is provided for the laser itself or it is providedfor the folder for supporting the substrate. As for the introduction ofthe laser beam, it is also sufficient that the light is introduced byusing not mirrors in the embodiment but an optical fiber. At that time,it is sufficient that an emitting edge portion of the optical fiber ismoved along each side of the substrate by using a guide or the like oran fluctuation is applied to the side of the emitting edge portion ofthe optical fiber, thereby fluctuating the incident angle.

As described in detail, according to the invention, since the laser beamis confined in the transparent material by using the total reflection asa physical critical phenomenon, the responses to the inspection light inthe ununiform and uniform portions of the transparent material becomecritical and the ununiformity appears as a very clear contrast, so thatthe ununiformity such as a fine scratch can be detected at a highsensitivity and it can be also detected at a high precision and a highspeed. Further, not only the ununiformity on the surface of thetransparent material but also the defect such as internal damage orstriae can be also detected.

The presence or absence, kind, and size of the ununiformity of thetransparent material are discriminated on the basis of the informationof the light leaked out of the surface of the transparent material, sothat a desired transparent material can be immediately extracted and theproducibility of the transparent material can be improved.

1. A method for inspecting an ununiformity of a transparent material byintroducing a laser beam therein; wherein said transparent materialcomprises at least one pair of total reflective surfaces arranged so asto face each other in such a manner that the laser beam propagates alongsaid pair of total reflective surfaces with being repeatedly and totallyreflected therebetween, at least one pair of turning surfaces arrangesso as to face each other in such a manner that the laser beam is turnedback when the laser beam is impinged thereagainst, and an introductorysurface for introducing the laser beam into said transparent materialdisposed in a manner to be sandwiched therebetween by at least one ofsaid total reflective surfaces and at least one of said turningsurfaces; said total reflective surfaces, said turning surfaces and saidintroductory surface being mirror-polished; and wherein when an opticalpath in said transparent material is uniform, the laser beam isintroduced in such a manner that the laser beam which propagates in thetransparent material and impinged on said total reflective surfaces andsaid turning surfaces totally reflects, and in an inspecting region ofthe transparent material sandwiched by one certain pair of said totalreflective surfaces and one certain pair of said turning surfaces, onecertain plane in said inspecting region filled with the laser beam bypropagating the laser beam in said transparent material is relativelymoved in the direction in which the inspecting region is filled with thelaser beam for said transparent material; and when an ununiform portionexists in the optical path of the laser beam which is introduced andpropagated in said transparent material, light which leaks out of saidtotal reflective surface and/or said returning surfaces is detected,thereby inspecting the ununiformity of the transparent material.
 2. Themethod of claim 1, wherein said transparent material is a glasssubstrate for an electronic device, and said total reflective surfacesare at least one pair of main surfaces which are parallel to each other,and said turning surfaces are at least one pair of end surfaces whichcross said main surfaces, and said introductory surface is a chamfersurface sandwiched by said main surfaces and said end surfaces.
 3. Themethod according to claim 2, wherein the glass substrate for anelectronic device is a glass substrate for a photo mask.
 4. The methodaccording to claim 2, where in the ununiformity is scratch, crack, staindue to an adhering foreign matter on a surface of the substrate, orinternal scratch, crack, bubble, striae of the substrate.
 5. The methodaccording to claim 2, wherein a wavelength of the laser beam is set to 1μm or less.
 6. A method for inspecting an ununiformity of a transparentmaterial by introducing a laser beam therein; wherein said transparentmaterial comprises at least one pair of total reflective surfaces so asto face each other in such a manner that the laser beam propagates alongsaid pair of total reflective surfaces with being repeatedly and totallyreflected therebetween, at least one pair of turning surfaces arrangedso as to face each other in such a manner that the laser beam is turnedback when the laser beam is impinged thereagainst, and an introductorysurface for introducing the laser beam into said transparent materialdisposed in a manner to be sandwiched therebetween by at least one ofsaid total reflective surfaces and at least one of said turningsurfaces; said total reflective surfaces, said turning surfaces and saidintroductory surface being mirror-polished; and wherein when an opticalpath is said transparent material is uniform, the laser beam isintroduced through a chamfer surface by changing an incident anglethereof within a range where the laser beam totally reflects on saidtotal reflective surfaces and said turning surfaces and repeats betweenat least one pair of end surfaces in such a manner that the laser beamwhich propagates in the transparent material and impinges on said totalreflective surfaces and said turning surfaces totally reflects, and thelaser beam is propagated so as to repeat between at least one pair ofsaid turning surfaces and the laser beam is spread in an inspectingregion which is formed by the propagation and is surrounded by saidtotal reflective surfaces and said turning surfaces; and when anununiform portion exists in the optical path of the laser beam which isintroduced and propagate in said transparent material, light which leaksout of said total reflective surface and/or said returning surfaces isdetected, thereby inspecting the ununiformity of the transparentmaterial.
 7. The method of claim 6, wherein said transparent material isa glass substrate for an electronic device, and said total reflectivesurfaces are at least one pair of main surfaces which are parallel toeach other, and said turning surfaces are at least one pair of endsurfaces which cross said main surfaces, and said introductory surfaceis a chamfer surface sandwiched by said main surfaces and said endsurfaces.
 8. The method according to claim 7, wherein the glasssubstrate for an electronic device is a glass substrate for a photomask.
 9. The method according to claim 7, where in the ununiformity isscratch, crack, stain due to an adhering foreign matter on a surface ofthe substrate, or internal, scratch, crack, bubble, striae of thesubstrate.
 10. The method according to claim 7, wherein a wavelength ofthe laser beam is set to 1 μm or less.